The invention relates to microwave assisted chemical reactions carried out at elevated temperatures and elevated pressures. In this context, the term “digestion” refers to the reaction of a sample with an aggressive acid (e.g., nitric, HNO3) at high temperatures and pressures. The combination of temperature and the strong acid tends to break most, and preferably all, of the chemical bonds in the sample to produce a liquid containing the constituent species, typically elements, of the sample. The liquid can then be analyzed for the presence and amounts of these elements.
Microwave systems are often used to accelerate the digestion process. Microwaves typically interact directly with the digestion acid and sometimes with the sample composition and thus in many cases microwave digestion can be carried out more quickly than digestion using conventional heat sources. Examples include, but are not limited to U.S. Pat. No. 5,420,039, U.S. Pat. No. 4,946,797, and U.S. Pat. No. 4,861,556.
Although digestion can be carried out using several different acids (e.g., sulfuric, nitric, phosphoric, hydrochloric, hydrofluoric, or perchloric), nitric acid offers advantages in some circumstances. In particular, nitric acid avoids forming insoluble compounds with many inorganic samples. Other acids (e.g., sulfuric and hydrochloric) are more likely to form such insoluble compounds during digestion reactions. Thus, nitric acid is often preferred for digestion because it produces a higher quality sample for analytical testing.
In order to digest in HNO3, however, many samples must typically be heated above the atmospheric boiling point of the acid; e.g., nitric acid boils at about 120° C., but many samples do not digest completely unless heated to at least about 200° C., and some samples require temperatures of 250-300° C. Thus, in order to reach higher temperatures, nitric acid digestion must be carried out in a pressurized environment, typically using vessels that can withstand pressures of several hundred pounds per square inch.
In order to prevent catastrophic failure at such pressures, most digestion vessels include some type of release capacity. These include rupture disks or diaphragms that break at a certain pressure (e.g., U.S. Pat. No. 5,230,865). Other digestion vessels will flex to create a small opening, for example between the body of the vessel and its lid, through which the excess pressure can escape (e.g., U.S. Pat. No. 6,287,526). Other systems are described in, for example, U.S. Pat. No. 5,948,307; U.S. Pat. No. 5,204,065; U.S. Pat. No. 5,264,185; U.S. Pat. No. 5,620,659 and EP0198675. These items are exemplary rather than exhaustive or limiting.
Such pressure release systems are effective for their intended purpose, but they lack precise control over the point at which they will release. Additionally, if the vessel re-seals itself, it does so at an arbitrary pressure rather than at a controlled pressure. As another factor, all vessels are ultimately limited in their pressure capacity.
To some extent, the gas-containing capacity of a vessel can be increased by increasing the vessel's size. Larger vessels, however, carry some corresponding disadvantages. They require, of course, larger instruments to accommodate them. From a safety standpoint, the total force within a vessel is a function of the pressure and the area defined by the vessel walls. Thus, larger vessels are subject to larger total forces and carry correspondingly higher risks of catastrophic failure.
Furthermore, in digestion systems where pressure is not released until the reaction is complete (and the vessel and its contents sufficiently cooled), the vessel volume must be sufficient to contain the sample, the acid, and the gases generated by the digestion reaction at the maximum digestion temperature.
Other pressure release systems attempt more sophisticated solutions. Légère and Salin, “Design and Operation of a Capsule-Based Microwave Digestion System,” Analytical Chemistry 1998, 70, pp. 5029-5036, describe an apparatus and system where a small (8.4 mm diameter, 25 mm length) polymeric gel capsule containing a sample is inserted into a Teflon™ tube. A digestion acid is then added to the tube and the tube is sealed. Microwaves are then applied to the tube, the gel capsule, the acid, and the capsule contents. The capsule breaks and the acid reacts with the sample. On a periodic basis, the application of microwaves is, however, stopped, the tube is proactively cooled with water, and excess gases are released. The technique is limited by the pressure capabilities of the tube and by the temperature at which the capsule material will digest. In other words, because the capsule breaks up and mixes with the digestion acid, the digestion temperatures must be maintained below those temperatures at which the capsule material would digest and add elements to the sample that would produce an improper analysis. According to Légère, polyacrylamide provides an appropriate capsule material, but contains trace quantities of iron, calcium, sodium, aluminum, and magnesium. Furthermore, polyacrylamide will tend to begin digesting at 230° C. As a result, the ongoing digestion reaction of the sample must be maintained sufficiently below 230° C. to avoid any digestion of the capsule and any consequent pollution of the sample results.
The Légère technique appears to have other disadvantages. As one, the reaction returns to atmospheric pressure on a repeated basis, thus effectively cooling the sample and reducing the temperature. As another disadvantage, both the described “flange valve” and the “squeegee” cleaning technique would appear to raise cross-contamination possibilities between and among digestion samples.